Backside illumination (BSI) image sensor chips are replacing front side illumination sensor chips for their higher efficiency in capturing photons. In the formation of the BSI image sensor chips, image sensors and logic circuits are formed on a silicon substrate of a wafer, followed by the formation of an interconnect structure on a front side of the silicon chip. The interconnect structure includes a plurality of metal layers including bottom metal layer M1 through top metal layer Mtop.
The wafer is then flipped over. A backside grinding is performed on the silicon substrate from the backside of the silicon substrate. A buffer oxide layer may be formed over the back surface of the remaining silicon substrate, and a first opening is formed to extend from the buffer oxide layer to stop at a shallow-trench isolation (STI) pad that is formed in the silicon substrate. A second opening is then formed inside the first opening to further etch the STI pad and the interlayer dielectric (ILD) that is directly under the etched portion of the STI pad, so that a metal pad in the bottom metal layer M1 is exposed. The second opening is smaller than the first opening. An aluminum copper pad is then formed in the first and the second openings and electrically coupled to the metal pad in metal layer M1. The aluminum copper pad may be used for bonding to the BSI chip.
It was found that conventional bond structures may suffer from film peeling during ball shearing tests. The metal pad in the bottom metal layer M1, which metal pad is bonded to the aluminum copper pad, may delaminate from the underlying etch stop layer. The peeling may be caused by the inferior adhesion between the metal pad and the etch stop layer, which is typically formed of silicon carbide.